Align websearcher to .Net 8 (#1078)

### Motivation and Context
websearcher is using an older version of .Net 8 than webapi and
memorypipeline

### Description
Align websearcher to .Net 8

### Contribution Checklist
- [ ] The code builds clean without any errors or warnings
- [ ] The PR follows the [Contribution
Guidelines](https://github.com/microsoft/chat-copilot/blob/main/CONTRIBUTING.md)
and the [pre-submission formatting
script](https://github.com/microsoft/chat-copilot/blob/main/CONTRIBUTING.md#development-scripts)
raises no violations
- [ ] All unit tests pass, and I have added new tests where possible
- [ ] I didn't break anyone 😄

Author: glahaye

plugins/web-searcher/WebSearcher.csproj

@@ -1,6 +1,6 @@
 <Project Sdk="Microsoft.NET.Sdk">
   <PropertyGroup>
-    <TargetFramework>net6.0</TargetFramework>
+    <TargetFramework>net8.0</TargetFramework>
     <AzureFunctionsVersion>v4</AzureFunctionsVersion>
     <OutputType>Exe</OutputType>
     <Nullable>enable</Nullable>